Plural layered metal repair tape

ABSTRACT

A base metal repair tape includes a first layer formed braze alloy bonded together with fibrillated polytetrafluoroethylene, a second layer formed from powdered base metal bonded together by fibrillated polytetrafluoroethylene and a third layer comprising a brazing alloy bonded together by fibrillated polytetrafluoroethylene. This is used to repair base metal by placing the first layer on the base metal and brazing the base metal powder so that the brazing alloy melts and diffuses into the base metal powder bonding it to the surface of the article. This permits the braze powder to be bonded to the base metal surface with minimal distance between the base powder particles. The number of alternating layers of base metal and braze alloy can be increased to increase the thickness of the repair. This can also be used to form small intricate parts.

RELATED APPLICATION

This application is a continuation of application Ser. No. 08/863,612,filed on May 27, 1997, now allowed U.S. Pat. No. 6,004,683 which isincorporated herein by reference in its entirety, which is acontinuation of Ser. No. 08/444,156, filed May 18, 1995, now abandoned,which is a continuation of Ser. No. 08/147,716, filed Nov. 4, 1993, nowabandoned, which is a continuation of Ser. No. 07/970,692, filed Nov. 4,1992, now abandoned.

BACKGROUND OF THE INVENTION

Metal parts, for example, those used in jet engines are generallyrequired to meet very precise tolerances. Damage to metal parts duringuse or during machining where a part is overmachined can prevent thepart from falling within the set tolerances and require that the part berepaired or replaced. To repair such damage, new metal must be brazed tothe surface of the part.

The new metal needs to have a composition similar to the base metal.Thus, diffusion braze fillers are combined with powder base metal toprovide a composition which brazes to the base metal at a temperaturelower than the melting point or softening point of the base metal.

Typically, this was done by forming a slurry which includes the powderedbase metal, powdered diffusion braze filler and a binder which could be,for example, a methacrylate binder, an alginate binder or the like.These systems provide acceptable results. However, well definedgeometries needed for some repairs were very difficult to obtain.

Further, slurries such as these are difficult to use. The binder systemmust be initially mixed. Then the precise amount of base metal anddiffusion filler must be combined. This has a very limited shelf life.It cannot be mass produced for sale and subsequent use. It must beprepared by the actual user which creates the potential problem of humanerror.

Also, the boron typically used in the braze alloy could localize orpuddle on the surface of the part. This weakens the base metal and candestroy the part. Slurries are also difficult to conform, resulting inpoor ability for large build-up repairs.

Also, with oxygen sensitive alloys such as those that include titanium,aluminum, hafnium, and chromium, heating above 800° F. can cause oxideformation. These oxides are not normally reducible in brazing furnaces.Most braze furnaces are designed to either operate in a vacuum or in ahydrogen atmosphere. However, there is frequently a trace amount ofoxygen remaining in the furnace that can react with these metals. Toavoid this problem, such alloys containing these oxygen sensitive metalsare nickel coated prior to base metal repair. This nickel precoating isundesirable simply because it requires an extra step or even two extrasteps frequently requiring masking of portions that are not to be nickelcoated.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a basemetal repair which is not a slurry and which does not require nickelprecoating.

Further, it is an object of the present invention to provide such a basemetal repair which provides precise dimensional repairs.

The present invention is a base metal repair tape which is a plurallayered tape. At least one inner layer is formed from base metal powderbonded together by a binder such as fibrillated polytetrafluoroethylene.At least two layers are diffusion braze alloy also bonded together by abinder such as fibrillated polytetrafluoroethylene. The outer layerssandwich the inner layer. These layers are bonded together and can beplaced directly on the repair area as a tape. During the thermal repaircycle, the diffusion braze alloy would melt and infiltrate the basemetal tape from both sides providing a repair which would essentiallykeep its dimensional integrity as well as have a greater per volumedensity of basis metal.

Where geometries are complex, stresses placed on the tape during repaircan cause the tape to lift away from the repair surface during thethermal processing. However using a three layered tape with a diffusionlayer contacting the repair surface enhances wetability and reduces thepossibility of tape movement. This lower layer is preferably maintainedrelatively thin to prevent damage to the repair surface. Since thislower layer is thin, there is little likelihood that boron will damagethe base metal.

This tape provides a cleaning ability to repair surface as well as thebase powder particles themselves. This can be extremely important wherethe base metal and the base metal powder of the repair contain highlevels of aluminum and titanium which are potential oxide formers andpotentially a threat to the success of the repair.

Further, the tape's cleaning ability reduces or eliminates the need fornickel plating over the part in the repair area due to the enhancedbrazability.

Further, the resulting brazed metal structure is enhanced by theseparation of the base metal powder and the diffusion metal powder. Thisis because using the current method, the distance between the base metalpowder particles is reduced due to the separation of the components inthe multi-layer tape. This creates a repair of more nearly part-likemechanical properties because of the higher base metal content per unitvolume.

By holding at brazing temperature or slightly below for an extendedperiod of time (e.g., 2 hours), the softening or melting point of therepair can approach the softening or melting point of the base metal,resulting in a higher quality repair, more closely approaching basemetal properties.

The reason for the improvement stems from the outward diffusion ofmelting point suppressant away from the repair area, significantlyreducing its concentration in the repair, and slightly raising it in thesurrounding base metal.

Further, the flexible and pliable nature of the tape allows repairs ofcomplex geometries to be easily addressed and promotes the easymanufacture of preforms for repairing multiple identical parts. Finally,after the polytetrafluoroethylene resin has been evacuated, the basemetal tape geometry structure remains intact providing for near drawingdimensioned preforms reducing timely and costly machining of the repairarea after repair thermal cycle.

In an alternate embodiment of the present invention, the tape can beformed with multiple alternating layers of base metal tape and diffusionbraze alloy forming a tape which is greater than one-half inch thick.This can be cut to size to form intricate parts which can be machined tosize and used.

These advantages of the present invention as well as others will beappreciated further in light of the following detailed description anddrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the present invention; and

FIG. 2 is a cross-sectional view of an alternate embodiment of thepresent invention.

DETAILED DESCRIPTION

The present invention is a laminated tape 11 used for repair of hardmetal surfaces 12, using a brazing technique. Hard metal surfaces orbase metals include, of course, all forms of stainless steel, as well asnickel, cobalt, titanium, and tungsten based superalloys such as Rene35, Rene 41, Rene 77, Rene 80, Rene 80H, Rene 95, Rene 125, Rene 142,Inconel 1606, Inconel 625, Inconel 713, Inconel 718, Hastelloy X, Waspalloy, Haynes 188, L605, X-40, MarM-509, and MarM-247.

As shown in FIG. 1, the laminate tape 11 is a three layered tapestructure. The intermediate layer 13 is a mixture of the base metalpowder and a binder. The outer two layers 14 and 15 are a mixture ofdiffusion braze alloy and binder.

There are a number of binders typically used to form braze tapes. Any ofthese binders typically used to form braze tape can be used. Thepreferred binder, however, is fibrillated PTFE. The fibrillated PTFEpolymer used in the process of this invention is a high molecular weightPTFE resin produced by emulsion polymerization. These PTFE polymers havea broad molecular weight range of about 10-20 million and arecommercially available products. Preparation of these polymers, which isdescribed in U.S. Pat. No. 2,510,112, U.S. Pat. No. 2,587,357, and U.S.Pat. No. 2,685,707, involves well known emulsion polymerizationtechniques wherein tetrafluoroethylene under pressure and watercontaining an emulsifying agent is reacted with a water soluble freeradical catalyst. The emulsion produced is coagulated, washed and thendried. The average particle size of the polymer is about 50-560 microns.Polymer having larger or smaller average particle size is operative.

The PTFE used to make the composition is commercially available fromE.I. DuPont and Company, Wilmington, Del. sold under the tradedesignation Teflon®6C. Each layer will have from 0.25 to about 25% ofthe fibrillated PTFE and preferably 1 to about 15% by weight. Thepreferred composition includes 3% PTFE by weight.

The second component of the intermediate layer 13 is the powdered basemetal itself which will make up the remainder of the composition.Microstructural improvements can be observed in the base metal powderlayer of the suggested multi-layer tape through the use of definedpowder sizes.

Densification of base metal powder is achieved by having a particle sizedistribution such that the interstitial spaces between the largerparticles are filled with smaller particles. Voided spaces to be filledby the diffusion alloy layer upon infiltration are minimized creating arepair area closer to base metal chemistry with a corresponding increasein desirable properties, for example, fatigue life. Also, while atheoretical distribution is desirable, a simple mixture of coarserpowder, for example, -80 mesh to +180 mesh together with a finer powder,for example, -400 mesh, can significantly increase the base metal layersdensity without the greatly added cost of an engineering grade ofpowder.

The outer layers 14 and 15 again are formed from two components, thebinder preferably fibrillated polytetrafluoroethylene and the diffusionbraze alloy. A diffusion braze alloy is typically an alloy similar incomposition to the base metal with the addition of a melt suppressant orit can be simply a braze alloy. There are many known commerciallyavailable braze alloys. The makeup of several of these compositions arelisted below:

    ______________________________________                                        1.            Boron    2.9                                                                  Nickel   92.6                                                                 Tin      4.5                                                    2.            Boron    3.0                                                                  Chromium 7.0                                                                  Iron     3.0                                                                  Nickel   83.0                                                                 Silicon  4.0                                                    3.            Chromium 19.0                                                                 Nickel   17.0                                                                 Silicon  10.0                                                   4.            Boron    1.8                                                                  Nickel   94.7                                                                 Silicon  3.5                                                    5.            Boron    0.8                                                                  Cobalt   Balance                                                              Chromium 19.0                                                                 Nickel   17.0                                                                 Silicon  8.0                                                                  Tungsten 4.0                                                    6.            Boron    2.75                                                                 Chromium 10.5                                                                 Nickel   50.3                                                                 Palladium                                                                              36.0                                                                 Silicon  0.5                                                    ______________________________________                                    

Such braze alloys can be purchased from companies such as Westgo,Praxair, and others.

To form the base metal intermediate layer 13 using the preferred binder,1 to 6% of the fibrillated PTFE (Teflon®6C) is combined with 94 to 99%(by weight) of the ground metal in a ball mill or other low shear mixersuch as a KD miller with kinetic dispersion or a vibratory mixer.

In a ball mill, the mixture is milled at about 200 rpm using 3/8 inchstainless steel balls. This is continued until the mixture changes froma powder to small agglomerated particles generally 10 to 40 minutes (25minutes normally). If this is continued too long, a breakdown of theagglomeration occurs resulting in material unsuitable for tape product.

The mixture is then separated from the steel balls and rolled betweenadjustable rollers to a desired thickness. Specifically, the mixture isrolled between pressure rollers in a first direction, then the formedsheet is folded between pressure rollers in a first direction, then theformed sheet is folded and the folded sheet is rolled in a directionwhich is 90° from the axis of the first rolling step, i.e., crossrolled. Each rolling step decreases the thickness of the sheet.Preferably, the sheet is separated from the rollers by an aluminum foilseparating sheet or other suitable material. This is continued until thedesired thickness and consistency is obtained.

Likewise, the outer or diffusion braze alloy layers 14 and 15 are formedin the same manner as the base metal layer 13 by combining 1 to 6%polytetrafluoroethylene with 99 to 94% diffusion braze alloy (byweight). This is then mixed in a ball mill separated from the steel shotand rolled to the desired dimension.

In use, the ratio of braze alloy to base metal powder in the overallcomposite 11 should be from about 0.1 to 1.0 up to 1.0 to 0.1 with about20% by weight of braze alloy and 80% base metal powder preferred.Increasing the ratio of braze alloy will decrease the braze temperaturebut the repair will be weaker. Also excess braze alloy will result intoo much flow reducing the ability of the repair to maintain its shape.

Generally, the three layers 13, 14, and 15 are each formed to athickness of about 0.001 to 0.06 inches or thicker and then are placedtogether with the diffusion braze alloy layers 14 and 15 sandwiching thebase metal layer 15. These are passed together through rollers to reducetheir overall thickness by about 50%. Thus, the final thickness of eachlayer will be about 1/2 the originally rolled thickness of theindividual layers, i.e. 0.005 to 0.030" each. This can then be cut tothe desired size for use. The thickness of the three layers along withthe ratio of binder to base metal or diffusion braze alloys in thelayers will control the ratio of diffusion braze alloy to base metal.

To use the composite 11, the damaged area of the base metal surface 12is covered with the repair tape with diffusion layer 15 against thesurface 12. The tape may be held to the base metal surface with anoption adhesive layer (not shown) such as Nicrobraze 200 or using a twosided adhesive tape purchased from 3M. The thickness of the layers isestablished to provide the amount of base metal needed for the repair.The thickness of the base metal layer will generally be 0.005 to 0.030"although this may change depending on the application. The thickness ofthe diffusing braze alloy layers should be 0.0025 to 0.015".

The object is heated to a temperature of at least about 800° F. to 2300°F. which causes the binder to evaporate and the braze alloy to melt andinfiltrate the base metal powder from above and below. For most nickeland cobalt alloys, at least 1750° F. is required. The braze alloy willthen, upon cooling, bond the base metal powder to the metal surface.

By holding at brazing temperature or slightly below for an extendedperiod of time, 30 minutes to 3 hours and preferably 2 hours, thesoftening or melting point of the repair can approach the softening ormelting point of the base metal, resulting in a higher quality repair,more closely approaching base metal properties.

The reason for the improvement stems from the outward diffusion ofmelting point suppressant away from the repair area, significantlyreducing its concentration in the repair, and slightly raising it in thesurrounding base metal.

With alloys of titanium, aluminum, chromium, and hafnium, heating to atemperature above 800° F. causes oxide formation. With the presentinvention, the hydrogen fluoride generated as the tape is heated removesthe oxides of these metals allowing a good braze joint without priornickel plating. This will be further appreciated in light of thefollowing example. Because microcrack cleaning is not required in thisrepair, the level of cleaning and reduced oxide formation from thestructural tape itself is sufficient for an adequate repair.

Multiple layer tape 21 can be prepared as shown in FIG. 2. In this FIG.2, the tape 21 has thirteen layers, seven layers 22 of the diffusionbraze alloy and six layers 23 of the base metal.

This composite tape again is made in the same manner as the tape shownin FIG. 1. The individual layers formed from the same proportions ofbase metal or diffusion braze alloy are combined with the fibrillatedpolytetrafluoroethylene and processed as previously described. Theindividual layers are combined together by running them through a press.With multiple layered tapes, the individual layers can be bondedtogether all at once or alternately can be combined two or four layersat a time from the middle portion towards the outer portion. This willcompensate for the pressure differential which will be mainly felt onthe exterior of the composite 21 as it passes through rollers.

The composite tape 21 shown in FIG. 2 in addition to providing a repairtape with extended thickness can also be used to actually manufactureparts or details of parts or assemblies. For example, sheet metal typeparts can be readily produced using the multi-layered system by cuttinga preform of the desired part from the tape 21 and processing thepreform through an appropriate thermocycle producing a coherent article.This method could be extremely beneficial where a part has an intricategeometry, but can be easily cut from a sheet of tape as opposed to acomplex and costly machining process.

As an example, the outer bands of jet engine nozzle assemblies which areabout 0.25 inches thick can be formed by producing a composite tapepreformed with Rene 80 powder and PTFE as the upper layer and a secondlayer of General Electric braze alloy, D15, bonded together by PTFE. Inthis embodiment, the base metal layer could be, for example, 0.25 inchesthick and the upper layer 0.063 inches thick. This would be formed intothe shape of the finished band with some allowance for shrinkage, placedon ceramic blocks which have been machined to the outer band radius ofthe actual parts and subjected to a thermal cycle. If a thicker partwere required, multiple layers of the base metal and diffusion alloy canbe conbined to form the tape.

Accordingly, the present invention provides the ability to provide largebase metal repairs on superalloys. It also is specifically beneficialfor making base metal repair on superalloys where the surface beingrepaired has a complex geometry. In addition, this invention lendsitself to the manufacture of intricate small parts and greatlysimplifies this production method.

A further advantage of the present invention is that it at leastpartially separates the braze alloy from the surface of the articlebeing repaired. Although a thin layer of the braze alloy may contact thesurface, the majority of the braze alloy is separated from the partsurface. The braze alloy itself can act to weaken the surface of themetal part. The preferred binder system for use in the present inventionis fibrillated polytetrafluoroethylene. However, many of the advantagesof the present invention will be appreciated employing any binder systemtypically used to form tapes for superalloy repair. However, by far thepreferred embodiment of the present invention employs the fibrillatedpolytetrafluoroethylene as a binder.

The invention itself, however, should be defined only by the appendedclaims wherein:
 1. A method of forming a super alloy base metal part foruse on a jet engine comprising:forming a flexible tape comprising brazealloy powder and oxygen sensitive superalloy base metal powder bondedtogether with 1%-6% by weight of polytetrafluoroethylene; forming saidtape to a desired size; heating said tape to a temperature of from about800° F. to about 2300° F.
 2. The method claimed in claim 1 wherein saidtape comprises a mixture of coarse powder-base metal and finerpower-base metal whereby interstitial spaces of said coarse base metalare filled with said finer base metal.
 3. The method claimed in claim 1wherein said tape is first heated to a temperature of about 500° F. forone hour to provide stress-release prior to subsequently reheating saidtape to a temperature of 800° F. to 2300° F.
 4. A method of repairing abase metal surface wherein said base metal surface comprises an oxygensensitive superalloy said method comprising:applying to said surface atape comprising base metal powder and a braze alloy powder wherein saidbase metal powder and said braze alloy powder are held together in acohesive layer by polytetrafluoroethylene; wherein said oxygen sensitivesuperalloy is selected from the group consisting of titanium, aluminum,chromium, and hafnium containing superalloys; positioning said tape onsaid surface and brazing said tape to said base metal surface in aninert environment.
 5. The method claimed in claim 4 wherein said inertenvironment is a vacuum.
 6. The method claimed in claim 4 wherein saidinert environment is hydrogen.
 7. The method claimed in claim 4 whereinsaid tape has a weight ratio of braze alloy to base metal powder of 5:95to 95:5 by weight.